1. Field of the Invention
The present invention generally relates to a method and system for the production of submicron materials, and more particularly to a method and system of synthesizing, in bulk quantities, nanosized powders, including nanocrystalline ceramics.
2. Description of Related Art
Ceramic materials are used in a wide variety of applications, and generally have excellent heat resistance, corrosion resistance, and abrasion resistance, as well as unique electrical or optical properties. Ceramic material, as used herein, generally refers to an oxide, nitride, boride or carbide of a metal, or a mixture thereof. Very fine ceramic powders are used in a large number of industrial processes to introduce or modify material properties. These materials can pose difficulties in sintering but, when they are converted to ultrafine particles, particularly submicron crystalline particles, numerous traditional problems are avoided. Accordingly, several processes have been devised for fabricating ultrafine, or submicron, crystalline materials, such as those of 1-500 nanometer size, referred to herein as nanosized or nanocrystalline.
Techniques for producing nanocrystalline materials generally fall into one of three categories, namely, mechanical processing, chemical processing, or physical (thermal) processing. In mechanical processes, fine powders are commonly made from large particles using crushing techniques such as a high-speed ball mill. There are several disadvantages with this approach. Sometimes metallic powders and highly reactive metals are combined with and subjected to such milling, which can pollute the material with a nanocrystalline alloy. Fragmented powders produced by mechanical processes can also result in particles of inconsistent shapes and sizes, and are often coarse and so not suited for high-performance applications.
With chemical processes, nanocrystalline materials are created from a reaction that precipitates particles of varying sizes and shapes, using a family of materials known as organometallics (substances containing combinations of carbon and metals bonded together). It is difficult, however, to produce ultrafine ceramics using organometallics without introducing excess carbon, or nitrogen (or both) into the final composition. Solution-gelation (sol-gel) ceramic production is similar to organometallic processes, but sol-gel materials may be either organic or inorganic. Both approaches involve a high cost of raw materials and capital equipment, limiting their commercial acceptance.
One of the earliest forms of physical, or thermal, processing, involves the formation and collection of nanoparticles through the rapid cooling of a supersaturated vapor (gas phase condensation). See, e.g., U.S. Pat. No. 5,128,081. In that example, a raw metallic material is evaporated into a chamber and raised to very high temperatures, and then oxygen is rapidly introduced. See also U.S. Pat. No. 5,851,507, in which a carrier medium is mixed with precursor material which is vaporized and subsequently rapidly quenched.
Thermal processes create the supersaturated vapor in a variety of ways, including laser ablation, plasma torch synthesis, combustion flame, exploding wires, spark erosion, electron beam evaporation, sputtering (ion collision). In laser ablation, a high-energy pulsed laser is focused on a target containing the material to be processed. The high temperature of the resulting plasma (greater than 10,000xc2x0 K) vaporizes the material so quickly that the rest of the source (any carrier and quenching gases) can operate at room temperature. The process is capable of producing a variety of nanocrystalline ceramic powders on the laboratory scale, but it has the great disadvantage of being extremely expensive due to the inherent energy inefficiency of lasers, and so it not available on an industrial scale.
The use of combustion flame and plasma torch to synthesize ceramic powders has advanced more toward commercialization. In both processes, the precursor material can be a solid, liquid or gas prior to injection into the flame or torch, under ambient pressure conditions. (the most common precursor state is a solid material). The primary difference between the two processes is that the combustion flame involves the use of an oxidizing or reducing atmosphere, while the plasma torch uses an inert gas atmosphere. Each of these processes requires relatively expensive precursor chemicals, such as TiCl4 for the production of TiO2 by the flame process, or TiC and TiB2 by the plasma process. A feature of both methods is the highly agglomerated state of the as-synthesized nanocrystalline ceramic powders. While for many applications the agglomeration of the powders is of little significance, there are situations where it is a shortcoming. Loosely agglomerated nanoparticle powders are produced in the combustion flame method of U.S. Pat. No. 5,876,683.
In the plasma process, reactants (feed materials) are delivered to a plasma jet produced by a plasma torch. See generally, U.S. Pat. Nos. 4,642,207 and 5,486,675. Alternatively, the feed material may be delivered to the plasma stream by arc vaporization of the anode. The anode is normally metallic but may be a metal-ceramic composite.
An improved plasma torch process is described in U.S. Pat. No. 5,514,349. That process can produce non-agglomerated ceramic nanocrystalline powders starting from metalorganic precursors, and uses rapid thermal decomposition of a precursor/carrier gas stream in a hot tubular reactor combined with rapid condensation of the product particle species on a cold substrate. Plasma torch processes, while gaining some limited commercial acceptance, are still energy inefficient and often involve materials which are extraneous to the products being produced. For example, in the ""349 patent, a working gas must be heated by the plasma arc, which is wasted energy. Also, since the product particles are suspended in the hot process gas stream, it is necessary to quench not just the particles but the process stream as well. The multiple gases used (the reaction gas, quench gas, and passivating gas) are either wasted, or must be separated for reuse.
A more recent development in vaporizing technology uses an electrothermal gun (electrogun). The electrogun is a pulsed power device which employs an electrode erosion phenomenon to vaporize one of the discharge electrodes (the cathode). The eroded metal vapor is subsequently ionized to form a dense plasma in which the high current discharge is sustained. The electrogun has a small length-to-diameter ratio and is designed to resist bore wall erosion. The vaporized metal exits the electrogun in a high-temperature, high-pressure, high-velocity jet. This jet is directed into a reactor filled with an appropriate atmosphere for reaction of the metal and quenching of the nanoparticles produced. Upon leaving the confines of the gun, the high-pressure jet expands rapidly. This expansion produces rapid cooling which promotes condensation of the vaporized material, thereby forming a spray of high-velocity metallic nanoparticles.
The electrogun uses batch processing powered by high-energy current pulses, while a plasma torch which operates continuously. Electrothermal synthesis, unlike plasma torch, heats the feed material directly, and does not produce any waste stream of process gases. The use of an electrogun is still somewhat energy inefficient, however, since it is necessary to chemically react the raw material to produce the nanoparticles, as opposed to merely physically converting another form of the material. It would, therefore, be desirable to devise a method of synthesizing nanocrystalline ceramics which is more energy efficient, and suitable for an industrial scale. It would be further advantageous if the method could reduce material cost.
It is therefore one object of the present invention to provide an improved method of producing nanosized ceramic particles.
It is another object of the present invention to provide such a method which is more energy efficient and uses a less expensive precursor material.
It is yet another object of the present invention to provide a method of synthesizing nanocrystalline ceramic powders, wherein the method includes the physical conversion of precursor ceramic material into a nanosized form.
The foregoing objects are achieved in a method of producing ceramic powder, generally comprising the steps of creating a plasma stream in a reactor vessel, and physically converting a ceramic precursor material into ceramic particles suspended in the vessel, using the plasma stream. The plasma stream is directed into an atmosphere of the vessel whose ambient conditions are selected to yield nanocrystalline ceramics. A metallic reactant may additionally be introduced into the vessel using the plasma stream, wherein the metallic reactant forms ceramic particles having the same composition as the ceramic particles of the physical converting step. The plasma stream may be created by delivering electrical current to an electrothermal gun. In one embodiment, the gun has a ceramic barrel which is eroded by the plasma stream. In another embodiment, the ceramic precursor material is injected as particulates into the plasma stream, the ceramic precursor particulates having a first size (e.g., micron or larger), and the ceramic particles suspended in the vessel have a second size which is substantially smaller than the first size (e.g., nanosized). These two embodiments may be combined as well.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.